FIG. 1A is a block diagram schematically illustrating a prior-art arrangement 10 for driving a CO2 (gas discharge) slab laser. Power amplifier 12 represents an output power amplifier stage of an RF power supply for driving the laser. The output impedance of such a power amplifier is typically 50 Ohms. The amplifier output is typically at frequencies up to about 150 megahertz (MHz) or greater. Such an amplifier stage can include a number of parallel combined transistor amplifier modules. The amplifier output is connected to electrodes (not shown) of a CO2 laser 18 via a 50 Ohm coaxial transmission cable (line) 14 and an impedance matching network 16. The transmission line can be considered as having an input port or node 13 and an exit port (or node) 15.
Cable 14 can be of any length and, accordingly, the power supply can be located remote from the laser. The impedance at entrance port 13 of the transmission line is essentially the same as an exit port 15 of the transmission line and is the same as the load impedance of the RF amplifier. The impedance of a CO2 laser scales inversely in relation to the power output of the laser. Accordingly as power output increases, the impedance of the amplifier must be matched to increasingly lower impedance at the laser, and the size, weight and cost of the impedance matching network increases accordingly.
Recent slab laser designs have included an RF power supply mounted directly on the laser such that a long transmission cable is not required. FIG. 1B schematically illustrates a prior-art arrangement 20 applicable with a directly mounted power supply. Here the RF power supply is connected by a transmission cable 22 having a length of one-quarter wavelength at a frequency typically between about 80 MHz and 100 MHz, directly to the CO2 laser. That length would be about 45 centimeters (cm) for a frequency of 100 MHz. In this example of FIG. 1B it is assumed that laser has an impedance of 4 Ohms which is about the impedance for a laser having an output between about 800 W and 1000 W. Transmission line 22 is assumed to have an impedance equal to the square root of the product 50×4 Ohms, i.e., about 14 Ohms. The impedance at entrance port 21 of the transmission line is 50 Ohms and the impedance at exit port 23 is 4 Ohms. In an arrangement such as the arrangement of FIG. 1B, if coaxial cable having a desired characteristic impedance is not commercially available, it will usually be possible to find cable having a sufficiently close match that a relatively simple inexpensive impedance matching network can be used to match the difference.
A disadvantage of the arrangement of FIG. 1B is that quarter-wave transmission line 22 is a resonant structure at a single frequency. A CO2 slab laser has a resonant frequency when the discharge of the laser is not lit (laser not operating) that is higher than the resonant frequency once the discharge is lit and the laser is lasing (operating). The operating resonant frequency can be as much as 25% or more lower than the non-operating frequency for a 1 KW laser. This makes it difficult, if not impossible, with the arrangement of FIG. 1B to apply the same power efficiently to the laser in both the un-lit and lit conditions.
Another disadvantage of the arrangement of FIG. 1B is that low impedance cables, for example cable having an impedance of less than 25 Ohms, are difficult to obtain commercially without a special order. Cables having an impedance of 50 or 75 Ohms are readily available and relatively inexpensive by comparison.
As laser power increases, and accordingly as RF power delivered to the laser increases (RF power is typically about ten-times greater than laser output power), both of the above described arrangements will encounter a problem with heating of the single transmission line along which the power is delivered. This will be true whether the transmission line is coaxial, a micro-strip line, or any other form of transmission line. This problem has been solved in the prior-art by an arrangement similar to that depicted in FIG. 2.
Here, a CO2 laser arrangement 24 includes a plurality of power amplifiers 12 (three in this example) delivering RF power via a corresponding plurality of 50-Ohm transmission lines 14. The output conductors of the lines are connected together and connected via an impedance matching network 16 to a CO2 laser 18. The three transmission lines can have the same (as illustrated) or different lengths. The impedance at each entrance port 13 is 50 Ohms and there is an impedance of 16.6 Ohms at a common output port 17 which is connected to the impedance matching network. This reduces demands on the design of the impedance matching network.
A problem with the arrangement of FIG. 2, encountered when driving a 1 kW laser, was that in order to effectively light the laser discharge it was necessary to make the transmission lines one half-wavelength long. This resulted in a bandwidth limitation similar to that described above with reference to FIG. 1B, which meant that power could not be delivered with maximum efficiency at both the un-lit and lit discharge frequencies. There is a need for an RF-power delivery arrangement for a CO2 laser than preferably does not require an impedance matching network, and has a sufficiently broad bandwidth that power can be delivered efficiently at both the un-lit and lit discharge frequencies of the laser.
Additional information about RF power supplies for gas lasers can be found in commonly owned U.S. Patent Publication 2008/0204134, the disclosure of which is incorporated herein by reference.